FIGURE 2-17 Area and volume coverage from orbit of space-based interceptors. SOURCE: Adapted from Figure 4.12 in David K. Barton, Roger Falcone, Daniel Kleppner, Frederick K. Lamb, Ming K. Lau, Harvey L. Lynch, David Moncton, et al., 2004, Report of the American Physical Society Study Group on Boost-Phase Intercept Systems for National Missile Defense: Scientific and Technical Issues, American Physical Society, College Park, Md., October 5, p. S60.

threats, and 2,000 or more to have a shot at the faster solid-propellant threats during their boost. Constellation sizing trades are discussed next.

Nations like North Korea and Iran lie between 25 and 42 degrees north latitude. Missiles fired from those countries toward the United States will burn out several hundred kilometers further north. So, the number of satellites required in the constellation is that number needed to always have at least one (and preferably two) close enough to reach a booster after it is launched—in all cases before booster burnout but (like all other boost-phase intercepts) before it reaches the velocity that would hit an area one wants to avoid—certainly Canadian or U.S. territory and, probably also, Russian or Chinese territory.

The laws of orbital mechanics mean that the minimum number of space-based interceptors must be in inclined orbits of at least 45 degrees. The population of SBIs is determined once again by the fly-out speed of the orbital interceptor and the time available, taking decision time into account, for the orbital interceptor to get from its storage orbit to the threat at its fly-out speed. This in turn defines the so-called absentee ratio—that is, the total number of interceptors required to be orbiting Earth every 90 min or so to assure that at least one is close enough to engage a single threat missile launched at any time. Even if it is assumed that North Korea would deploy only slow liquid-fueled missiles like those that it currently uses, several hundred satellites would be required in the constellation

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